The interests of the Department of Biochemistry at this Institute are focused on: (a) pteridine, folate and vitamin B12 metabolism, (b) folate and Methotrexate transport, (c) mitochondrial structure and function, (d) mitochondrial genetics and (e) heme binding proteins. During the past two decades the various projects in each of these areas have progressed from recording gross observational phenomena, through studies with partially purified extracts, to the current isolation of single proteins or protein complexes, which are completely homogeneous when examined by such techniques as polyacrylamide electrophoresis, isoelectric focusing, ultracentrifugation, gel filtration and absorbance spectrophotometry. It is now possible, therefore, to investigate these purified materials at the molecular level and to relate their mechanisms of action and ligand binding to structure. A prerequisite for the determination of such structures is a knowledge of their amino acid composition. With access to an amino acid analyzer it will become possible to determine the compositions of: (a) rat liver dihydropteridine reductase and phenylalanine hydroxylase; (b) vitamin transport proteins from Lactobacillus casei and L1210 cells; (c) the proteins of mitochondrial ATP asynthetases (Complex V), and other oxidative phosphorylation complexes from both beef heart and Neurospora crassa; (d) cytochrome P450, and (e) B12 binding proteins from hog pylorus, gastric juices, and serum. Additionally, it will be possible to identify amino acid residues vital to protein-protein interactions and/or substrate binding of these proteins, hemopexin, and chemically modified dihydrofolate reductases. Most of these proteins, hemopexin, and chemically modified dihydrofolate reductases. Most of these proteins are routinely obtained in homogeneous form in this Department and the determination of their amino acid compositions will contribute greatly to their understanding of their biological functions in vivo.